This invention generally relates to ophthalmic lenses, and more particularly to ophthalmic lenses having multiple optical powers or focal lengths.
Multifocal ophthalmic lenses are often worn to help individual focus on near objects. To elaborate, each eye contains a natural lens that is used to focus images on the retina of the eye. In a person with normal eyesight, this lens of the eye is naturally shaped to focus the images of distant objects on the retina, and the eye lens is bent in order to focus the images of near objects on the retina. This adjustment of the eye lens to focus the images of objects at various distances on the retina is referred to as accommodation.
With many people, the accommodation of the eyes is inadequate, a condition referred to as presbyopia. For example, the accommodation of the human eye naturally diminishes with age, so that many people develop presbyopia as they age. Also, because of cataracts, many people have their natural lenses removed and replaced with artificial intraocular lenses, which have no ability to change or adjust inside the eye. Inadequate accommodation may be corrected by spectacles or other lenses having a number of different regions with different optical powers. With such spectacles, the wearer can shift his or her line of vision so that an object is observed through the portion of the spectacle having the appropriate optical power needed to focus properly the image of the object.
Heretofore, efforts to provide contact lenses that have multiple focal lengths, either to correct for presbyopia or for other reasons, have not been completely satisfactory.
One prior art approach to providing multifocal contact lenses is to provide a user with a different contact lens for each eye. One contact lens is provided with near power -- that is, the lens is designed to focus on near objects -- and the other contact lens is provided with distance power -- that is, the lens is designed to focus on distant objects. This approach, referred to as monovision, utilizes the fact that, under certain circumstances, the brain can discriminate between separate, competing images received by the two eyes, and can accept an in-focus image while rejecting a competing out-of-focus image. Monovision can correct or alleviate presbyopia without requiring complex lens designs or manufacture. However, monovision has important disadvantages in that it results in losses in binocular image summation and in substantial loss of depth perception, referred to as binocularity.
Another example of multifocal contact lenses that utilizes the ability of the brain to discriminate between competing images is disclosed in U.S. Pat. No. 4,923,296. This patent describes a contact lens system which comprises a pair of contact lenses, each having equal areas of near and distance optical power. The lens for one eye has an upper half with near optical power and a lower half with distance optical power, while the lens for the other eye has an upper half with distance optical power and a lower half with near optical power. When the lenses are used together, as described in this patent, the lenses provide at least partial clear images in both eyes and, through suppression by the brain of the blurred images, allow alignment of the clear image to produce a focused image.
An additional prior art approach to providing multi-focal contact lenses employs the principal that the size of the pupil of the eye changes depending on the intensity of the light incident on the eye; and specifically, the pupil contracts in response to brighter light. Thus, when a person is wearing contact lenses, then, in response to brighter light, the light that is focused on the retinas of the eyes comes through smaller areas of the contact lenses.
For example, U.S. Pat. No. 4,890,913 describes a bifocal contact lens comprised of a number of annular zones having different optical powers. The objective of this lens design is to maintain an approximately equal division between near and distance optical powers at all times and regardless of pupil diameter, and this requires between six and twelve zones of differing optical powers on the lens.
A further attempt at providing a bifocal contact lens is described in U.S. Pat. No. 4,704,016. The lens disclosed in this patent also attempts to maintain, at all times and regardless of pupil diameter, an approximately equal division between near and distance powers.
Another approach to producing a multifocal corrective eye lens involves the use of diffractive optics. One disadvantage of this approach, though, is a deficiency in vision at low light levels. To elaborate, in a diffractive design, only about 40% of the light incident on the lens is used for near vision with another 40% being used for far vision. The remaining 20% of the incident light is not used for either near or far vision, but rather is lost to higher orders of diffraction and to scatter effects. These 40% values represent the best theoretical case and, in practice, even less light is available due to difficulties encountered in manufacturing these lenses. Difficulty of manufacture in general represents another disadvantage of diffractive lenses since the diffractive surfaces of these lenses must be produced within tolerances that are on the same order of magnitude as the wavelength of light. For the reasons discussed above, although simple multifocal refractive lens systems, such as monovision, are somewhat understood, more complex schemes for such lens systems are primarily theoretical.
U.S. Pat. Nos. 5,002,382 and 5,024,517 disclose complementary pairs of contact lenses having two or more corrective optical powers in opposite configurations. Both of the lenses in each pair, however, contain only two zones of different optical power.
A more practical and improved approach to providing a multi-focal ophthalmic lens is described in copending application Ser. No. 07/827,199 filed on Jan. 28, 1992. This application discloses a multifocal ophthalmic lens having a central zone that is part of one of the multifocal segments of the lens. The boundary between the segments of this lens is defined by an arcuate path such as a semi-circle and both ends of this path are on the adjoining perimeter of the near and distance segments. This design eliminates from the central optical axis the segment boundaries including the central junction point.
Lenses made according to the above-described application are functional under certain illumination conditions with some patients. Nevertheless, it is believed that the general level of satisfaction with multifocal ophthalmic lenses may be improved. With current multifocal lens designs, patients may have problems with competing images under high levels of illumination, and may have problems reading under medium-to-low illumination conditions In addition, at night, patients may have problems caused by haloes appearing around light sources, a problem that may be particularly troublesome in night driving situations.